The inner surface of a pneumatic rubber tire is typically composed of a rubbery, elastomeric composition designed to prevent or retard the permeation of air and moisture into the carcass from the tire's air chamber. The portion of the tire containing such inner surface is often referred to as an inner liner. Inner liners have been used for many years in tubeless, pneumatic vehicle tires to retard or prevent the escape of air used to inflate the tire, thereby maintaining tire pressure.
Rubbers which are relatively impermeable to air are often used as a major portion of such inner liners. These can include butyl rubber and halobutyl (e.g. chlorobutyl and bromobutyl) rubbers. U.S. Pat. No. 3,808,177 discloses other polymers which may also be relatively impermeable.
The inner liner is normally prepared by conventional calendering or milling techniques to form a strip of uncured compounded rubber. Typically this strip is the first element of the tire to be applied to a tire building drum, over and around which the remainder is built. When the tire is cured, such inner liner becomes an integral, co-cured part of the tire. Other techniques are used in some cases; for example, U.S. Pat. No. 3,299,934 discloses application of a cured polyurethane in the form of a solution to the inner surface of the tire. The polyurethane is a reaction product of a partially diisocyanate-modified polyester prepolymer with a further quantity of diisocyanate and a small amount of castor oil.
Other references disclosing tires having coatings of low air permeability on the inner surface of a tire include Japan published patent application No. 47-31761, published Aug. 15, 1972 (vinylidene chloride copolymers, polyesters and polyamides) and United Kingdom published patent application No. GB 2 023 516 A (polyvinylidene chloride).
Rubber composition based inner liners add both weight and cost to the manufacture of a tire. Relatively thick inner liner coatings are generally necessary to obtain the desired degree of air retention; the added weight and cost are largely attributable to the thickness of coating required.
Photopolymerizable compositions, or photopolymers, have been reported as being useful in the manufacture of various items including printing plates, printing inks and paints among others, but not as coatings for forming pneumatic tire inner liners. Photopolymers comprising polyurethane formed by reaction of a polyhydroxy compound with a diisocyanate and having methacrylate end groups are disclosed, for example, in U.S. Pat. Nos. 3,891,523; 3,912,516; and 4,057,431. These compositions are disclosed as being photopolymerizable, e.g. polymerizable by exposure to actinic light. Nothing is disclosed or taught in any of these references as to either the air permeability or the elongation properties of the resulting photocured resins.
Curing of photopolymers by means of ultraviolet light or electron beam radiation has also been described in the literature, "Radiation Curing: An Introduction to Coatings, Varnishes, Adhesives and Inks", published by the Association for Finishing Processes of the Society of Manufacturing Engineers, Publications/Marketing Division, Dearborn, Mich., described curing by both ultraviolet (UV) and electron beam (EB) radiation, and describes in general terms equipment for both types of radiation processes.
Actinic light generators or lamps are well known and are presently used in photopolymerization. By way of example, U.S. Pat. No. 4,485,332 describes an electrodeless lamp which contains mercury and which emits ultraviolet radiation when excited with microwave energy. Lamps of this type are described as being useful in the curing of coatings or inks by photopolymerization.
Other types of ultraviolet lamps for curing are also known.
U.S. Pat. No. 4,434,372 to Cleland describes an electron beam scattering apparatus and its use in curing tire inner liners.